Knee ligament disruptions are common injuries resulting from accidents or traumatic shock sometimes experienced during sporting activities, such as skiing, football and soccer. Since the ligaments of the knee guide and constrain knee motion, their disruption can result in degenerative changes and subsequent injuries to the knee if the damage is not treated soon after the initial injury. Reconstructive surgery is commonly employed to restore the knee to its preinjured condition and function.
Leg strength and muscle mass, however, are invariably sacrificed during recovery from surgery, while motion and muscle tension are limited. This effect is especially pronounced in the quadriceps muscles. The amount of lost quadriceps mass is directly related to the length of time the muscles are immobilized or limited during such activities as walking, squatting, stair climbing and cycling. Thus, it is necessary to undertake an effective exercise regimen as early as possible to restore normal muscle mass.
It is known that muscles are strengthened and their mass increased by repetitive contraction of the muscles under tension. When the muscle contracts, however, it shortens, thereby causing joint motion via the connective tendons which can stress the ligaments connecting the bony joint segments. When the damaged knee is reconstructed, as when a cruciate ligament is replaced or reattached to its bony terminus, the repair must be protected from stress until biological mending is near completion. Thus, conventional modes of quadriceps and hamstring exercises are prohibited during this period of recovery.
It is further known that contraction of the quadriceps muscles causes anterior displacement of the tibia relative to the femur when the knee is in early flexion, i.e., from full extension, which is defined as 0.degree., up to an extension angle of about 70.degree.. Anterior displacement of the tibia is even more pronounced when the anterior cruciate ligament is torn. Thus, it is apparent that contraction of the quadriceps muscles during early knee flexion stresses or loads the anterior cruciate ligament. Studies also indicate that the anterior cruciate ligament is not stressed by contraction of the quadriceps muscles during late knee flexion, i.e., 70.degree. to 90.degree.. Contraction of the quadriceps muscles, however, causes posterior displacement of the tibia when the knee is in late flexion. Thus, the posterior cruciate ligament is apparently stressed by contraction of the quadriceps muscles during late flexion, but not during early flexion. Hamstring muscle contraction is also thought to cause tibial displacement and corresponding stressing of the cruciate ligaments when the knee is at different angles of extension.
Recognition of these ranges of extension angles suggests an exercise modality which spares the surgically reconstructed cruciate ligament from loading or stress during contraction of the hamstring or quadriceps muscles. As such a leg muscle exercise device is needed which effectively operates within these safe exercise modes. More particularly, a leg muscle exercise device is needed which provides for safe early exercises to minimize lost muscle mass and strength during surgical rehabilitation. Further, a reliable means of measuring or controlling the force extended by the muscle without supervision is needed during voluntary or electrically simulated contractions of the muscles.